PRE-CAM5RIAN AND PALEOZOIC ERAS 159 



Accordingly we see that Crossopterygii of Devonian times possessed a 

 mechanism capable of developing into the respiratory system needed by 

 terrestrial vertebrates — an example of preadaptation (pp. 12-16). In 



CEPHALASPIS - 

 AN OSTRACODERM 



FIG. 8.14. An ostracoderm (armored fish), Cephalaspis; length one foot or less. (By 

 permission from Comparative Anatomy, by Neal and Rand, p. 24. Copyright, 1936. 

 McGraw-Hill Book Company, Inc.) 



many of the modern bony fishes, on the other hand, the air bladder has no 

 opening to the pharynx; gas pressure in the bladder is regulated through 

 action of the blood system. Thus the bladder serves as a swim bladder, a 



FIG. 8.15. A plocoderm (acanthodian) fish (Climatius) from the De- 

 vonian; length about 3 inches. (From Romer, Vertebrate Paleontology, 

 University of Chicago Press, 1 945, p. 41 .) 



hydrostatic organ enabling the fish to adjust to varying pressures at dif- 

 ferent depths. 



Ancestral crossopterygians also shared with lungfishes another feature 

 useful for respiration in air. The external nostrils, instead of opening into 

 blind pouches as they do in most fishes, connected to openings in the roof 



